(a) Field of the Invention
The present invention relates to a large-aperture macro lens system and, more particularly, to a large-aperture macro lens system which is capable of photographing an object at the infinite distance up to an object at a distance corresponding to the photographing magnification of 1.times. and which has a focal length of 50 mm and large aperture ratio, i.e., F/2.8.
(b) Description of the Priot Art
For photographic lens systems in general, aberrations are corrected so that the lens system displays the best performance when it is focused on an object at the infinite distance. On the other hand, for macro lens systems, a photographing magnification about 1/10.times. is used as the standard photographing magnification, and aberrations are corrected so that the performance of the lens system becomes favourable when it is focused on an object at a distance corresponding to the standard photographing magnification.
However, in case of such macro lens systems, it is impossible to satisfactorily correct the variation of aberrations which occur according to the variation of the photographing magnification and, when the lens system is focused on an object at a distance shorter than the object distance corresponding to the standard photographing magnification, the lens system causes spherical aberration, coma, etc. largely. Especially, in case of a lens system such as, for example, Gauss type lens system which is arranged to be symmetrical in respect to the stop, there is general tendency that, as the photographing magnification is made higher, spherical aberration to be caused by rays of intermediate heights is undercorrected and spherical aberration to be caused by rays of the maximum height is overcorrected. As a result, the quality of image decreases considerably, and it is impossible to effectively utilize the strong point of a large aperture macro lens system. This is because paraxial rays, which go out from the lens components arranged in front of the stop and which are converging rays in respect to an object at the infinite distance, gradually come to a state close to the afocal state according to the variation of the photographing magnification and, at a photographing magnification about 1.times., said paraxial rays become diverging rays. When this is examined from the view point of coefficients of aberrations, spherical aberration of the third order is undercorrected while spherical aberration of the fifth and seventh orders is considerably overcorrected. As a result, flare is caused and the quality of image decreases.
The above-mentioned problem is evident from the coefficients of spherical aberration of Embodiment 1 of a known macro lens system, which is disclosed in Japanese published examined patent application No. 20006/83, when the lens system as a whole is advanced. Said coefficients of aberration are as shown below.
______________________________________ SA3 SA5 SA7 Total ______________________________________ Infinite Distance -0.109 +0.076 +0.027 -0.006 -0.5X -0.246 +0.241 +0.089 +0.084 -1.0X -0.201 +0.427 +0.171 +0.397 ______________________________________
In the table shown in the above, values of respective coefficients of aberration correspond to values of lateral aberration on the image surface to be caused by paraxial rays of the maximum height. Here, reference symbol SA3 represents the coefficient of spherical aberration of the third order, reference symbol SA5 represents the coefficient of spherical aberration of the fifth order, and reference symbol SA7 represents the coefficient of spherical aberration of the seventh order.
The floating method is known as a means for correcting the variation of aberrations which occurs at the time of focusing on an object at a short distance as described in the above. Macro lens systems to which the floating method is applied are disclosed in Japanese published unexamined patent applications Nos. 28038/80, 107208/81, etc. Each of the above-mentioned known macro lens systems comprises two lens groups, i.e., a front lens group (a first lens group) located in front of a stop, and a rear lens group (a second lens group) located in rear of the stop. Another known macro lens system, which is disclosed in Japanese published unexamined patent application No. 192916/82, is arranged to comprise a front lens group (a first lens group) located in front of a stop, a rear lens group (a second lens group) located in rear of the stop, and a third lens group having positive refractive power which is added on the image side of said rear lens group, and still another known macro lens system, which is disclosed in Japanese published unexamined patent application No. 228220/84, is arranged to comprise a front lens group (a first lens group) located in front of a stop, a rear lens group (a second lens group) located in rear of the stop, and a third lens group having negative refractive power which is added on the image side of said rear lens group. Each of the known macro lens systems described so far is arranged that floating is carried out, when focusing on an object at a short distance, by varying an airspace between respective lens groups in the monotone increasing pattern.
The known macro lens systems described so far are respectively arranged that, when focusing on an object at the shortest object distance from the state that the lens system is focused on an object at the infinite distance, an airspace between respective lens groups is varied in the monotone increasing pattern so as to increase the heights of paraxial rays on a converging surface contained in the second lens group and to thereby cause undercorrected spherical aberration so that overcorrected spherical aberration to be caused by rays of the maximum height is offset by said undercorrected spherical aberration, and the variation of spherical aberration to be caused when focusing on an object at a short distance is thereby corrected. However, the above-mentioned method has disadvantages described below. That is, up to a photographing magnification about 1/2.times., it is possible to correct spherical aberration favourably. However, at a photographing magnification about 1.times., undercorrected spherical aberration occurs largely, flare is caused in the central portion of the image surface and, as a result, the quality of image becomes unfavourable. Moreover, the displacement of the image surface to be caused when the stop is stopped down becomes large. Furthermore, the advancing amount of the lens system at the time of focusing becomes large, and it is impossible to make the lens system compact.
Still another known macro lens system is disclosed in Japanese published unexamined patent application No. 100115/85. Said known macro lens system comprises a first lens group having positive refractive power, a second lens group having positive or negative refractive power, and a third lens group having positive refractive power and is arranged to perform floating, when focusing on an object at a short distance, by varying the airspace between the first and second lens groups in the monotone increasing pattern and varying the airspace between the second and third lens groups in such pattern that said airspace increases and, then, decreases.
Generally, when a macro lens system is focused on an object at a distance corresponding to the photographing magnification about 1.times., the lens system advancing amount required becomes equal to the focal length of the lens system in the state that the lens system is focused on an object at the infinite distance. Therefore, even when the lens system is arranged compactly, the product provided with a lens system advancing mechanism does not become compact in most cases. In case of the above-mentioned known macro lens system disclosed in Japanese published unexamined patent application No. 100115/85, both of the first and second lens groups have positive refractive powers. Therefore, the photographing magnification available cannot be made higher than 1/2.times., the lens system advancing amount for focusing is large, and it is impossible to obtain a compact lens system.